Tectonically, Peninsular Malaysia forms part of the Sunda Shield. Its Triassic fold-mountain belt, the spine of the Peninsula, continues from eastern Burma through Thailand, Peninsular Malaysia, the Banka and Billiton Islands, and eastwards into Indonesian Borneo.  Thick formations of Lower Carboniferous limestone in central Pahang and carbonaceous shales with limestone lenses in east Pahang provide the earliest indications of the formation of the basin to the east of the Main Range. Sedimentation here was typically shallow marine and, in Kelantan, was probably continuous till the Early Permian.

The Japanese invasion of Malaya and British Borneo in late 1941, which culminated in the humiliating British surrender in Singapore two and a half months later, shattered Western colonial supremacy and unleashed the forces of incipient nationalism. Although the British were able to resume their authority in the region after the collapse of Japan in 1945, they faced an entirely new political situation and those circumstances forced them to adopt new policies. As a result the Straits Settlements were dissolved. Pulau Pinang and Melaka were joined with the Malay States of the Peninsula to form a new Malayan Union. Singapore became a separate crown colony and so did both Sarawak and British North Borneo in place of the former Brooke and Chartered Company regimes. Labuan was joined to British North Borneo. These new arrangements met with considerable Malaysian opposition